“I’m proud to stand with him, because I think he has done a great job, and I think right now, for example, [he] is one of the greatest allies we have in working with Congress to make sure we get real, meaningful health-care reform,” said Rivera, who heads the Service Employees International Union.

When he was president of the SEIU’s Local 1199 here four years ago, Rivera backed Freddy Ferrer over Bloomberg.

Asked if that was a mistake, Rivera ducked by saying, “I don’t want to review the past. I just want to look forward.”

Thompson dismissed Rivera’s endorsement, as well as that of former Clinton White House Chief of Staff John Podesta, by pointing out neither can actually vote here.

“I think there are more Democrats from outside the city who have endorsed Mike Bloomberg than Democrats inside the city,” said Thompson.

Thompson could also take heart in a new Survey USA survey that showed him trailing by just 8 points, 51-43 percent.